Suction modulation valve for refrigerant system with adjustable opening for pulse width modulation control

ABSTRACT

A pulse width modulation control is provided for a suction modulation valve in a refrigerant system. An intentional small “leakage” path is maintained through the suction modulation valve to ensure that the pressure inside the compressor shell does not decrease below a safe reliability threshold but, at the same time, does not exceed a certain value, which would cause the refrigerant system to operate inefficiently, when the pulse width modulation control has moved the suction modulation valve to a closed position. The size of this minimum “leakage” path is continuously adjusted to ensure that the optimum pressure inside the compressor shell is maintained regardless of the evaporator pressure and other operating conditions.

BACKGROUND OF THE INVENTION

This application relates to a refrigerant system, in which a suctionmodulation valve (or other type of a valve which has a small controlledopening in the closed position) is provided with pulse width modulationcontrol to adjust refrigerant system capacity. A minimum opening size ofthe suction modulation valve is maintained to ensure that suctionpressure inside a shell of the compressor located downstream of thesuction modulation valve does not decrease below a specified value.However, this minimum opening size is adjusted in response to systemoperating conditions to ensure that the suction pressure within thecompressor is close to the allowable minimum, and is not undesirablyhigher.

Refrigerant systems are known, and are utilized to condition a secondaryfluid. As an example, an air conditioning system cools and dehumidifiesair being delivered into a climate controlled environment. Refrigerantsystems generally include a compressor compressing refrigerant anddelivering that refrigerant through a discharge line to a first heatexchanger. From the first heat exchanger, refrigerant passes through anexpansion device and then through a second heat exchanger. Therefrigerant is then returned to the compressor.

Under various conditions, a refrigerant system may provide excess ofcapacity to cool or heat a secondary fluid supplied to a climatecontrolled environment. A number of methods are known for reducing thecapacity of the refrigerant system.

One known method of reducing capacity is to provide a pulse widthmodulation control for a suction valve located upstream of thecompressor to control the amount of refrigerant moving from the secondheat exchanger to the compressor. In pulse width modulation control fora suction valve, the valve is rapidly cycled (opened and closed) tolimit the amount of refrigerant flowing to the compressor. This in turnlimits the refrigerant amount compressed in the compressor andrefrigerant flow circulating throughout the refrigerant system,resulting in a capacity reduction for the refrigerant system, andproviding more efficient operation.

One challenge with regard to such operation is that the pressure withinthe compressor shell must not be reduced below a specified limit definedby compressor reliability considerations. As a rough guideline, it isdesirable to maintain a pressure within the compressor shell of at least1 psia. However, when the suction modulation valve is completely closedduring pulse width modulation control cycle, sometimes, the pressurewithin the compressor shell can decrease below this specified minimumpressure. Under such circumstances, sparking can occur at the terminalsfor the compressor motor, which can lead to terminal damage. Thisphenomenon is known as a “corona discharge” effect, and is undesirable.

Thus, it is known in the prior art to provide a minimum “leakage”opening for the suction valve, while it would be otherwise closed duringpulse width modulation cycle, to prevent compressor suction fromentering a deep vacuum region. Also, in another approach, a branchbypass line, containing a small internal diameter capillary tube or asmall orifice, around the pulse width modulation valve has been proposedin the past to prevent compressor suction from going into deep vacuum byproviding an alternate small “leakage” path for refrigerant flowing intothe compressor. While the prior art does provide good control ofcapacity, the “leakage” opening is typically sized to ensure that thesuction pressure in the compression shell exceeds the specified minimumpressure at all operating conditions.

However, the downstream pressure inside the compressor shell, when thesuction valve is in the closed position, changes substantially for aconstant size opening, depending on the pressure upstream of theopening. The evaporator pressure can vary by at least an order ofmagnitude, depending on the operating conditions of the refrigerantsystem. Therefore, under high pressure operating conditions at theevaporator, in the prior art, the suction pressure inside the compressorwould also be much higher then what can be considered desirable for theminimum pressure in order to avoid the “corona discharge” effect. Havingthe suction pressure well above this threshold is undesirable, since itdecreases the efficiency of the refrigerant system operating in a pulsewidth modulated mode. Thus, the prior art could not effectively controlthe suction pressure inside the compressor to be just above theacceptable threshold for all operating conditions, while at the sametime avoiding the “corona discharge”.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, a control for a suctionmodulation valve operates the suction modulation valve using pulse widthmodulation control to reduce refrigerant system capacity. When the valveis in the closed position, the control varies the size of the minimum or“leakage” opening in the valve, depending on the refrigerant systemoperating conditions. In a disclosed embodiment, the controllingrefrigerant system operating condition would be a pressure upstream ofthe suction modulation valve. This pressure is typically associatedwith, and closely approximated by, the pressure inside the evaporator.The evaporator pressure can be measured by one of the sensors, and theregistered value is related to a desired minimum opening of the suctionmodulation valve to achieve a minimum desired pressure within thecompressor shell. As known, the smaller the opening of the valve, thelarger the pressure drop through the valve, therefore, for the sameupstream evaporator pressure, the downstream compressor suction pressurecan be controlled by varying the size of this opening. In this manner,the prior art problem of having suction pressure far above the minimumthreshold pressure within the compressor shell, under high evaporatorpressure conditions, during periods of time when the suction modulationvalve is in the closed position, is eliminated.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a refrigerant system incorporating thepresent invention.

FIG. 2 shows the operation of a pulse width modulation control in theprior art.

FIG. 3A and FIG. 3B show a problem with the prior art systems.

FIG. 4 is a chart explaining the feature of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A refrigerant system 20 is illustrated in FIG. 1. The refrigerant system20 incorporates a compressor 22 compressing refrigerant and deliveringit downstream to a condenser 24. Refrigerant from the condenser 24passes through an expansion device 26, and then to an evaporator 28.Refrigerant from the evaporator 28 passes through a suction modulationvalve 30 and back to the compressor 22. As is known, a control 34 forthe suction modulation valve 30 may provide a pulse width modulationcontrol to rapidly change the size of the opening through the valve 30between open and closed positions, in order to limit the amount ofrefrigerant passing from the evaporator 28 to the compressor 22. In thismanner, a reduced capacity during part- load operation for therefrigerant system 20 can be achieved.

As shown in FIG. 2, the refrigerant system capacity is cycled between amaximum (fully open suction modulation valve) and minimum value (suctionmodulation valve closed with a minimum opening) over time, such that theaverage capacity is less than the full-load capacity without the pulsewidth modulation control.

FIG. 3A and FIG. 3B explain shortcomings in the prior art. As mentionedabove, some “leakage” path is typically maintained across the suctionmodulation valve to ensure that a relatively small amount of refrigerantdoes reach the compressor 22, and such that a minimum suction pressureis maintained within a compressor shell 52. As explained above, a motor50 for a compressor pump unit 51 is received within the compressor shell52. If the pressure within the compressor shell 52 becomes unduly low,then a “corona discharge” effect can occur, which is undesirable. Forthis reason, a refrigerant “leakage” path is typically provided toprevent the compressor from entering into a deep vacuum region. However,the size of this minimum “leakage” path has typically been designed toensure that the pressure will never drop below the specified minimumpressure (e.g., 1 psia) for all operating conditions. For example, ifthe minimum expected upstream pressure, P_(UPSTREAM), is equal to 30psia, then the size of the minimum opening is designed to be such thatthe downstream pressure, P_(DOWNSTREAM), at the suction modulation valveclosed position, is at 1 psia, as shown in FIG. 3B. However, at 100 psiaP_(UPSTREAM) pressure value, for the same amount of opening for thesuction modulation valve 30, the P_(DOWNSTREAM) is about 6 psia, asshown in FIG. 3A, even though, for the most efficient operation, itwould have been desirable to also have 1 psia pressure downstream of thesuction modulation valve.

FIG. 4 shows a chart of pressure downstream (P_(DOWNSTREAM)) of thesuction modulation valve versus pressure upstream (P_(UPSTREAM)) of thesuction modulation valve for three different minimum opening sizesthrough the pulse width modulation valve (e.g., opening A1, opening A2,and opening A3) when the valve is in the closed position. The larger theopening, the larger is the P_(DOWNSTREAM) pressure for the sameP_(UPSTREAM) pressure. As indicated in FIG. 4, A1 is the largest minimumopening size, A3 is the smallest minimum opening size, and A2 minimumopening size falls between A1 and A3 opening sizes. As can be seen inFIG. 4, when the valve has the largest minimum opening size A1, thedownstream pressure, P_(DOWNSTREAM), is equal to 1 psia, when theupstream pressure, P_(UPSTREAM), is equal to 30 psia. Further, for thesame opening A1, P_(DOWNSTREAM) is equal to 6 psia, when P_(UPSTREAM) isequal to 100 psia. However, what is desirable is to have 1 psiadownstream pressure, P_(DOWNSTREAM), regardless of the upstream pressureP_(UPSTREAM). This P_(DOWNSTREAM) pressure of 1 psia can be achieved byhaving the adjustable minimum suction modulation valve opening, namelythe minimum suction modulation valve opening needs to be at A1, whenP_(UPSTREAM) pressure is equal to 30 psia, and the minimum suctionmodulation valve opening needs to be at A3, when P_(UPSTREAM) pressureis equal to 100 psia.

As can be appreciated from FIG. 1, a pressure sensor 32 can bepositioned upstream of the suction modulation valve 30 to measure theupstream pressure, P_(UPSTREAM). Another sensor 44, can be positioneddownstream of the suction modulation valve 30 to measure the pressuredownstream of the suction modulation valve 30, P_(DOWNSTREAM) (thisdownstream pressure corresponds to and typically closely approximatesthe suction pressure inside the compressor shell). From the graph inFIG. 4, a desired area “A” of the minimum suction modulation valveopening, which provides a desired 1 psia minimum downstream pressure,P_(DOWNSTREAM), while the suction modulation valve is in the closedposition, can be selected. It has to be noted that exemplary FIG. 4 onlyshows three curves for different area “A” openings, and a more precisegraph is to be developed with a larger number of more closely spacedlines corresponding to areas “A”, such that the desired area “A” can beaccurately selected by interpolating between the lines corresponding toareas shown on this graph. The control 34 thus not only drives thesuction modulation valve 30 to have a pulse width modulation movementbetween opened and closed positions, but also adjusts the minimumopening for the suction modulation valve 30 depending on operatingconditions (and the pressure upstream P_(UPSTREAM) of the suctionmodulation valve 30, in particular) to maintain 1 psia P_(DOWNSTREAM)pressure regardless of the upstream pressure P_(UPSTREAM). Thus, thepressure within the compressor shell 52 can always to be maintainedclose to the minimum pressure (e.g., 1 psia), rather than being higherthen desired, causing irreversible efficiency losses in operation of therefrigerant system 20.

Instead of developing a graph as shown in FIG. 4, the refrigerant system20 can have a feedback control, where the amount of minimum opening forthe pulse modulation valve 30 can be adjusted based on pressure detectedby a sensor 44, that is measuring the downstream pressureP_(DOWNSTREAM). If the sensor 44 measures the value of P_(DOWNSTREAM) tobe substantially higher than 1 psia, when the pulse width modulationvalve 30 is in the closed position, then the minimum opening size forthe pulse width modulation valve 30 is reduced. In case the downstreampressure, P_(DOWNSTREAM), is trending to drop below 1 psia, then theminimum opening size for the suction modulation valve 30 is increased.The control 34 can also operate in a learning mode, or in a mode when itlearns what amount of opening is needed to maintain the downstreampressure P_(DOWNSTREAM) nearing the vicinity of 1 psia, with respect tothe upstream pressure P_(UPSTREAM).

The graph presented in FIG. 4 is exemplary and shown for illustrationpurpose only, as the exact shape of the curves would depend on theparticular compressor size and type, refrigerant type, etc. In additionto relying on the measurement of upstream pressure, P_(UPSTREAM), otherparameters can be measured to fine tune the establishment of therequired minimum opening area of the pulse width modulation valve 30 inthe closed position (such as temperature upstream and downstream of thevalve, etc.). While a scroll compressor is used to illustrate thisinvention, other compressor types would fall within the scope of theinvention, including, for example, rotary, screw, and reciprocatingcompressors. This invention can be applied to various types of systemsand can include refrigeration container and truck-trailer systems,supermarket installations, residential air conditioning and heat pumpsystems, and rooftop units. Lastly, as mentioned above, other valvetypes capable to adjust minimum opening size would be within the scopeand can equally benefit from the invention.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A refrigerant system comprising: a compressor, said compressordelivering refrigerant to a first heat exchanger, refrigerant passingform said first heat exchanger through an expansion device and to asecond heat exchanger, refrigerant passing from said second heatexchanger through a suction valve and back to said compressor; and acontrol for said suction valve, said control operable to rapidly cyclesaid suction valve between open and closed positions to adjust thecapacity of the refrigerant system, and said suction valve maintaining aminimum opening area in the closed position, said control selecting saidminimum opening area to ensure a pressure within a shell for saidcompressor approximates a minimum predetermined pressure when saidcontrol has moved said suction valve to its closed position.
 2. Therefrigerant system as set forth in claim 1, wherein said suction valveis a suction modulation valve.
 3. The refrigerant system as set forth inclaim 1, wherein said minimum pressure is between 0.5 psia and 3 psia.4. The refrigerant system as set forth in claim 1, wherein said minimumopening is selected by said control based on pressure associated withsaid second heat exchanger.
 5. The refrigerant system as set forth inclaim 4, wherein said pressure is measured at a location downstream ofsaid second heat exchanger, and upstream of said suction valve.
 6. Therefrigerant system as set forth in claim 4, wherein a relationship isdetermined between said pressure and said minimum opening for saidsuction valve to ensure that the pressure within said compressor shellapproximates the minimum pressure, and said relationship being utilizedby said control to select said minimum opening.
 7. The refrigerantsystem as set forth in claim 1, wherein said minimum opening is selectedby said control based on pressure measurements indicative of saidpressure within said compressor shell.
 8. The refrigerant system as setforth in claim 7, wherein said control decreases said minimum opening ifthe said pressure within said compressor shell is higher than desired.9. The refrigerant system as set forth in claim 7, wherein said controlincreases said minimum opening if the said pressure within saidcompressor shell is lower than desired.
 10. A method of operatingrefrigerant system comprising the steps of: (1) providing a compressor,said compressor delivering refrigerant to a first heat exchanger,refrigerant passing form said first heat exchanger through an expansiondevice and to a second heat exchanger, refrigerant passing from saidsecond heat exchanger through a suction valve and back to saidcompressor; and (2) rapidly cycling said suction valve between open andclosed positions to adjust the capacity of the refrigerant system, andsaid suction valve maintaining a minimum opening area in the closedposition, said control selecting said minimum opening area to ensure apressure within a shell for said compressor approximates a minimumpredetermined pressure when said control has moved said suction valve toits closed position.
 11. The method as set forth in claim 10, whereinsaid suction valve is a suction modulation valve.
 12. The method as setforth in claim 10, wherein said minimum pressure is between 0.5 psia and3 psia.
 13. The method as set forth in claim 10, wherein said minimumopening is selected by said control based on pressure associated withsaid second heat exchanger.
 14. The method as set forth in claim 13,wherein said pressure is measured at a location downstream of saidsecond heat exchanger, and upstream of said suction valve.
 15. Themethod as set forth in claim 13, wherein a relationship is determinedbetween said pressure and said minimum opening for said suction valve toensure that the pressure within said compressor shell approximates theminimum pressure, and said relationship being utilized by said controlto select said minimum opening.
 16. The method as set forth in claim 10,wherein said minimum opening is selected by said control based onpressure measurements indicative of said pressure within said compressorshell.
 17. The method as set forth in claim 16, wherein said controldecreases said minimum opening if the said pressure within saidcompressor shell is higher than desired.
 18. The method as set forth inclaim 17, wherein said control increases said minimum opening if thesaid pressure within said compressor shell is lower than desired.